Among anion exchange resins, those with acrylic polymer backbones are known for having higher exchange capacity and greater fouling resistance than styrenic resins. The weakly basic, acrylic resins prepared from conventional acrylic esters, however, are difficult to rinse after regeneration. One reason for this rinse difficulty is that trace amounts of carboxylic acid functionality remain in the functionalized resin, where they retain cations from the regenerant, and release them gradually into the rinse water. Prolonged rinsing eventually reduces the cations to a reasonable level, but this takes excessive time and rinse water.
The prior-art solution to this high-rinse-volume requirement has been to use macroreticular acrylic resins, but these resins have significantly lower anion exchange capacity than the high-rinse-volume, acrylic resins.
Corte et al., in U.S. Pat. No. 3,544,488, describe a copolymer of acrylonitrile with aliphatic, including cycloaliphatic, crosslinkers containing two or more unconjugated vinyl groups; such a copolymer resists hydrolysis much better than acrylonitrile copolymers crosslinked with divinylbenzene or other, aromatic crosslinkers. In the Corte reference these copolymers were used to prepare only cation exchange resins. Bufton et al., in U.S. Pat. No. 3,423,336, disclose preparation of weakly basic anion exchange resins by functionalizing acrylonitrile copolymers with anhydrous polyamines. The copolymers are crosslinked with conventional crosslinkers such as esters and polyvinyl aromatic monomers, and suffer from the stability problems discussed by Corte et al. Abrams et al., in U.S. Pat. No. 3,389,096, disclose a similar reaction to that of the Bufton reference, but functionalization occurs in the presence of water. As the copolymer is the same as Bufton's, the disadvantages are the same.